linux-kernel - Re: [PATCH 0/3] allow zram to use zbud as underlying allocator

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Date:	Thu, 1 Oct 2015 09:52:16 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Vitaly Wool <vitalywool@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Dan Streetman <ddstreet@...e.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	김준수 <iamjoonsoo.kim@....com>,
	Gioh Kim <gioh.kim@....com>
Subject: Re: [PATCH 0/3] allow zram to use zbud as underlying allocator

On 09/30/2015 05:46 PM, Vitaly Wool wrote:
> On Wed, Sep 30, 2015 at 5:37 PM, Vlastimil Babka <vbabka@...e.cz> wrote:
>> On 09/25/2015 11:54 AM, Vitaly Wool wrote:
>>>
>>> Hello Minchan,
>>>
>>> the main use case where I see unacceptably long stalls in UI with
>>> zsmalloc is switching between users in Android.
>>> There is a way to automate user creation and switching between them so
>>> the test I run both to get vmstat statistics and to profile stalls is
>>> to create a user, switch to it and switch back. Each test cycle does
>>> that 10 times, and all the results presented below are averages for 20
>>> runs.
>>>
>>> Kernel configurations used for testing:
>>>
>>> (1): vanilla
>>> (2): (1) plus "make SLUB atomic" patch [1]
>>> (3): (1) with zbud instead of zsmalloc
>>> (4): (2) with compaction defer logic mostly disabled
>>
>>
>> Disabling compaction deferring leads to less compaction stalls? That indeed
>> looks very weird and counter-intuitive. Also what's "mostly" disabled mean?
>
> Not that I'm not surprised myself. However, this is how it goes.
> Namely, I reverted the following patches:
> - mm, compaction: defer each zone individually instead of preferred zone

Oh, I see. Then you didn't disable compaction defer logic, but made it 
coarse again instead of per-zone. Which means that an allocation that 
can be satisfied from Normal zone will use the Normal zone's deferred 
state to decide whether to compact also DMA and DMA32 zones *within the 
same allocation attempt*. So by reverting the patch you might indeed get 
less compact_stall (and success+failure) counts, but each stall will try 
to compact all three zones. With individual defer, some stall might be 
just for DMA32, some just for Normal, and the total number might be 
higher, but the compaction overhead should be better distributed among 
all the attempts. Looking at your latencies, looks like that's working fine:

>
> The UI is blocked after user switching for, average:
> (1) 1.84 seconds
> (2) 0.89 seconds
> (3) 1.32 seconds
> (4) 0.87 seconds

Average for (2) vs (4) is roughly the same, I would guess within noise.

> The UI us blocked after user switching for, worst-case:
> (1) 2.91
> (2) 1.12
> (3) 1.79
> (4) 1.34

The worst case is actually worse without individual defer, because you 
end up compacting all zones in each single stall. With individual defer, 
there's a low probability of that happening.

> - mm, compaction: embed migration mode in compact_control

This probably affects just THPs.

> - mm, compaction: add per-zone migration pfn cache for async compaction

Hard to say what's the effect of this.

> - mm: compaction: encapsulate defer reset logic

This is just code consolidation.

> ~vitaly
>

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